With the progressive miniaturization of individual circuit elements in integrated circuits, the resolution limitations of optical microlithography have become increasingly apparent and difficult to overcome. Hence, considerable effort currently is being expended to develop a practical microlithography apparatus that utilizes an energy beam offering prospects of greater resolution than obtainable with optical microlithography. One approach in this regard involves using a charged particle beam (e.g., electron beam or ion beam) instead of a light beam as an energy beam.
Charged-particle-beam (CPB) microlithography methods (and associated apparatus) are of several types. One type involves "direct writing" in which an electron beam "draws" a pattern directly on a substrate (e.g., semiconductor wafer) without using a reticle or mask. Another type, broadly classified as CPB "projection" microlithography, utilizes a reticle (defining a pattern) that is projected onto the substrate using a charged particle beam.
Most current efforts directed to making a practical CPB projection microlithography apparatus concern a partial one-shot approach, also termed a "divided transfer" approach utilizing a "divided-transfer" microlithography apparatus. In the divided-transfer approach, a pattern field on the reticle is divided into portions, termed subfields, and each subfield is exposed individually and thus transferred to the substrate. The entire pattern field defined by the reticle is transferred subfield-by-subfield as the reticle and substrate are moved relative to each other. Under such an exposure scheme, extremely accurate alignment of the reticle and substrate with each other is critical.
Further regarding the partial one-shot approach, the substrate is mounted on a substrate stage and the reticle is mounted on a reticle stage. The stages are movable relative to each other. Accurate alignment of the reticle and substrate, as noted above, requires that the following be executed and controlled extremely accurately: (1) detection of the relative positions of the reticle stage and substrate stage, (2) detection of the relative positions of the reticle and substrate, (3) moving the reticle stage and substrate stage relative to each other, and (4) correcting the positional relationship of the stages, as well as of the reticle and substrate, as required during exposure. Detection and correction of relative positional relationships of the transfer-receiving side (substrate side) and the transfer-originating side (reticle side) are referred to herein as positional "alignments."
In one conventional alignment method, an electron beam is used to determine one or more of the foregoing positional relationships using index marks on the reticle stage or the reticle itself, and corresponding index marks (fiducial marks) on the substrate stage or on the substrate itself. The positions of the reticle stage and substrate stage relative to each other are detected from a backscattered-electron (BSE) signal produced by scanning an electron beam, that has passed through an index mark on the reticle or reticle stage, over a corresponding fiducial mark(s) on the substrate or substrate stage. An amount of relative angular displacement (rotation) of the direction of travel of the reticle stage relative to the direction of travel of the substrate stage is determined, e.g., by detecting the relative positions of two separated marks, as shown in FIGS. 7(a)-7(b).
More specifically, in the conventional method, the substrate stage is moved to a position at which the marks can be detected (i.e., a position at which a BSE signal from the substrate or substrate stage can be detected), and the substrate stage is held stationary at that position. Next, the reticle stage is moved to a respective position allowing detection of the first mark on the reticle or reticle stage (i.e., a position at which the first mark can be illuminated by the electron beam, which propagates through the first mark to a respective fiducial mark on the substrate or substrate stage), and the position of the first mark is detected. Then, the reticle stage and substrate stage are moved to respective positions allowing detection of the second mark on the reticle or reticle stage (i.e., a position at which the second mark can be illuminated by the electron beam, which propagates through the second mark to a respective fiducial mark on the substrate or substrate stage), and the position of the second mark is detected. During detection of the first and second marks, the respective positions of the reticle stage and substrate stage are monitored and detected using respective interferometers. Using data obtained by the reticle-stage interferometers for reference, an angle .theta..sub.1 of rotation between the direction of travel of the reticle stage and a straight line connecting the two marks on the reticle stage (or reticle) is calculated from the detected amounts of movement of the reticle stage and the respective detected positions of the two marks (FIG. 7(a)). Similarly, using data obtained by the substrate-stage interferometers for reference, an angle .theta..sub.2 of rotation between the direction of travel of the substrate stage and a straight line connecting the two marks on the substrate stage (or substrate) is calculated (FIG. 7(b)).
Then, the reticle stage is moved to place the two marks on the reticle stage (or reticle) within a primary deflection range of the electron beam. The reticle stage then is stopped. Similarly, the substrate stage is moved to place the two marks on the substrate stage (or substrate) within the primary deflection range. With the stages in their respective positions, mark detections are performed to determine the positional relationship between the first and second marks on the reticle stage (or reticle) and the first and second marks on the substrate stage (or substrate). An angle of rotation .theta..sub.3 between a straight line connecting the first and second marks on the reticle stage (or reticle) and a straight line connecting the first and second marks on the substrate stage (or substrate) is ascertained from the results of these calculations. By combining these results with the results summarized above, the relative rotation between the direction of travel of the reticle stage and the direction of travel of the substrate stage is determined (FIG. 7(c)).
From the foregoing method, by which respective marks on the stages are aligned relative to the directions of travel of the respective stages, rotation amounts are calculated between the reticle and the direction of travel of the reticle stage, and between the substrate and the direction of travel of the substrate stage. Respective rotations in the substrate and reticle can be calculated from these results.
Because the alignment method summarized above includes respective movements of the two stages to place the two marks within the main deflection range of the electron beam to perform mark detection, several problems arise. The first problem is that mark detection is performed by deflecting the beam. This is not a problem whenever the measurement equipment is adjusted adequately and sufficient accuracy is achieved at whatever position to which the beam is deflected. However, an electron-optical system normally is not always adjusted adequately. For example, adjustment normally is inadequate whenever the apparatus is first turned on. In such a situation, accurate mark detection must be performed with the respective marks situated at a position at which the beam is not deflected (i.e., at the axis of the optical system). This is because the accuracy of mark detection is poor whenever mark detection is performed with the respective marks situated at a beam-deflection position.
The second problem is that, whereas calculation of relative rotations is made by performing a mark-position calculation for each of the detected marks, if the calculation is performed simply to make an alignment bringing the relative rotation amount to zero, and not to obtain a measurement of the actual amount of rotation, then calculating the actual amount of rotation for each mark must be performed repeatedly, which is extremely inefficient.